In recent years, display device develops at a high speed. Thin film transistor, serving as a driving element of the display device, has developed from a-Si (non-crystal silicon) thin film transistor to LTPS (low temperature poly-silicon) thin film transistor and oxide thin film transistor, etc.
The oxide thin film transistor is widely used at present, employs an oxide semiconductor as an active layer, and has advantages of high mobility, high on-state current, better property as a switch, high uniformity and suitable for the display device that requires fast response and larger current, such as the display device with high frequency, high resolution or large size, and the organic light-emitting display device.
Structure of current oxide thin film transistor includes two types: etch stop layer (ESL) type and back channel etch (BCE) type. In the ESL type oxide thin film transistor, an etch stop layer is fabricated on an oxide semiconductor layer, and a via hole is etched in the etch stop layer so as to connect source and drain electrodes and the oxide semiconductor layer via the via hole. In the ESL type oxide thin film transistor, the oxide semiconductor is well protected; however, a patterning process for forming the etch stop layer is added, so that the fabrication process of the thin film transistor is complicated and the cost is increased. In addition, in the ESL type oxide thin film transistor, a channel size of the thin film transistor is limited because a distance between via holes is limited by process capability for forming the via holes and the source drain metal layer, which limits increase of aperture ratio of the display device that employs the oxide thin film transistor and limits application of the oxide thin film transistor in high PPI (pixels per inch) products. In the BCE type oxide thin film transistor, the patterning process for forming the etching a stop layer is omitted, but the oxide semiconductor layer is not protected; therefore, it is of high requirement for etching process and the oxide semiconductor layer easily becomes unstable. Hence, the BCE type oxide thin film transistor is not be widely used.